Yes but as the ULA whitepaper pointed out there are some challenges to doing so in orbit. For one, the lack of gravity makes it hard to pump fuel. You can spin the tanker to achieve fuel settling but now you are both spinning bodies. Then there is handling cryogenic propellants in space, you get nominal bleed off from warming and again ULA had a really interesting design of an internal combustion engine using the bleed off of Hydrogen and Oxygen as a chiller pump. But suffice it to say, its not as easy as it is flying a jet behind a slow re-purposed jetliner, and doing that is already difficult.
Evidently this problem is solved, because liquid fuel rockets have been working fine in space since the 60s. (Liquid fuel rockets have their propellants pumped into the chamber.)
> nominal bleed off from warming
Launch the tanker just beforehand. There won't be time for the fuel to bleed off.
I didn't know about ullage motors, thanks for the link. The link also says that only very tiny accelerations are needed - so this shouldn't be a big problem for a tanker. Like I hypothesized, the problem has been solved.
Bearing in mind that the ullage motors need to be of a type that won't suffer from ullage problems. That implies they cant use the main motor fuel.
The saturn 5 used small solid rockets as ullage motors. Alternatively you could use externally pressurised bladder tanks for a range of non-cryogenic fuels. Cold gas or possibly h2o2 as a monopropellant come to mind. Either option has a comparatively low specific impulse, so even running them at very low overall accelerations for extended time periods is likely to be cost-prohibitive.
I like the suggestion elsewhere of spinning the tank. You could also dock, then spin the whole combination ship.
If you abandon the fuel station idea, you could just take the extra fuel tank with you, transferring fuel to your internal tank during a main engine burn, then abandoning the empty tank in a wierd orbit.
Do the fuels have to remain in a cryogenic state in space? Can much larger containers with warmer fuel be used once the fuel is in orbit? I have no idea if this makes any sense.
A major problem is pressure. As the fuel heats up and boils off the pressure increases and the tanks have to bleed it off or they'll burst eventually. Also the engine design for cryogenic stages assume a liquid fuel and moving enough gaseous fuel and oxidizer isn't in the current designs.
That's a good question. A big bag of fuel hanging outside is a fine place to store it if you don't care about temperature or pressure. You would need some kind of mechanical means to squish the bag to get the fuel out though, so maybe an accordion with a motorized retractor?
I think you are underestimating expansion ratios (ratio of volume of liquid and gas at reasonable pressure). The ratio for water vapour at sea level is ~1000x. The "large bag" might end up being so large that it's weight is a significant fraction of the weight of the fuel it contains.
